VPN with Radius - ssl-login-fail (but auth. on server is succesfull)

Hello,

i use FG50E v6.0.1 and have problem with authentication on radius server. I need to authenticate users, who are in specific AD group. 

When i try connect with forticlient:

1) Event viewer on server say, that Network policy server gives acces to user (successful audit)

2) VPN events on fortigate say: "Action ssl-login-fail / Reason sslvpn_login_unknown_user"

Could you help me with that please?

Thank you!

Best Regards,

Michal Stoklasa